The mechanisms by which oxygen and chemical radiosensitizing agents act in irradiated mammalian cells will be studied, both independently and interactively. Aspects of the nature of radiation sensitization will be examined in detail in CHO cells under conditions where both hypoxic cell sensitizers and low levels of oxygen are present. In the clinical application of sensitizers such as the nitroimidazoles, e.g., misonidazole, to cancer radiotherapy, it is likely that there will be an important subpopulation of tumor cells that contain a low concentration of oxygen, but are not radiobiologically hypoxic. The effects of sensitizers on such cells will be determined in this project. The nature of the interaction of oxygen and chemical sensitizers has yet to be identified: the agents may act independently or in a complex additive manner. Also to be investigated is the possibility of an additive or synergistic effect between radiosensitization and the specific hypoxic cell toxicity recently observed with many sensitizer compounds. This study will evaluate these two factors which are of importance relative to the optimum application of sensitizers in fractionated radiotherapy. Experiments to be performed using high dose rate techniques will provide information on the kinetics and mechanism of sensitization and of the related process of radiolytic oxygen depletion, known to occur in mammalian cells containing low levels of oxygen, which may play an important role in radiotherapy for solid tumors. It is expected that these data will be useful in identifying site(s) of action of oxygen and other sensitizers and therefore aid in the formulation of the molecular mechanism for radiosensitization. Also to be studied, using the high dose rate radiation capability, are several other modifiers of radiation damage, namely "shoulder-modifying" sensitizing agents, and compounds believed to alter the indirect action damage to DNA. These studies will provide information on the mechanisms of radiation damage, especially pertaining to the effects of oxygen and radiosensitizers, which are important current concerns in radiobiology and cancer radiotherapy.